Certain carriers are utilized for transporting and storing batches of silicon wafers or magnetic disks before, during, and after processing of the disks or wafers. The wafers are processed into integrated circuits and the disks are processed into a magnetic storage disks for computers. Wafers when used herein refers to silicon wafers, magnetic substrates, and the like.
The processing of wafer disks into integrated circuit chips often involves several steps where the disks are repeatedly processed, stored and transported. Due to the delicate nature of the disks and their extreme value, it is vital that they are properly protected throughout this procedure. One purpose of a wafer carrier is to provide this protection. Additionally, since the processing of wafer disks is generally automated, it is necessary for disks to be precisely positioned relative to the processing equipment for the robotic removal and insertion of the wafers. A second purpose of a wafer carrier is to securely hold the wafer disks during transport.
Carriers are generally configured to axially arrange the wafers or disks in slots, and to support the wafers or disks by or near their peripheral edges. The wafers or disks are conventionally removable from the carriers in a radial direction upwardly or laterally. Carriers may have supplemental to covers, bottom covers, or enclosures to enclose the wafers or disks. There are a number of material characteristics, which are useful and advantageous for wafer carriers depending on the type of carrier and the particular part of the carrier at issue.
During processing of semiconductor wafers or magnetic disks, the presence of or generation of particulates presents very significant contamination problems. Contamination is accepted as the single biggest cause of yield loss in the semiconductor industry. As the size of integrated circuitry has continue to be reduced, the size of particles which can contaminate an integrated circuit has also become smaller making minimization of contaminants all the more critical. Contaminants in the form of particles may be generated by abrasion such as the rubbing or scraping of the carrier with the wafers or disks, with the carrier covers or enclosures, with storage racks, with other carriers, or with the processing equipment. A most desirable characteristic of a carrier is therefore a resistance to particle generation upon abrasion, rubbing, or scraping of the plastic molded material. U.S. Pat. No. 5,780,127 discusses various characteristics of plastics which are pertinent to the suitability of such materials for wafer carriers. Said patent is incorporated by reference.
Carrier materials should also have minimal outgassing of volatile components as these may leave films which also constitute a contaminant which can damage wafers and disks. The carrier materials must have adequate dimensional stability, that is rigidity, when the carrier is loaded.
Dimensional stability is necessary to prevent damage to the wafers or disks and to minimize movement of the wafers or disks within the carrier. The tolerances of the slots holding wafers and disks are typically quite small and any deformation of the carrier can directly damage the highly brittle wafers or can increase the abrasion and thus the particle generation when the wafers or disks are moved into, out of, or within the carrier. Dimensional stability is also extremely important when the carrier is loaded in some direction such as when the carriers are stacked during shipment or when the carriers integrate with processing equipment. The carrier material should also maintain its dimensional stability under elevated temperatures, which may be encountered during storage or cleaning
Conventional carriers used in the semiconductor industry may develop and retain static charges. When a charged plastic part comes into contact with an electronic device or processing equipment it may discharge in a damaging phenomena known as electrostatic discharge (ESD). Additionally, statically charged carriers may attract and retain particles, particularly airborne particles. Also static buildup on carriers can cause semiconductor processing equipment to automatically shut down. It is most desirable to have a carrier with static dissipation characteristics to eliminate ESD and to avoid attracting particles.
Trace metals are a common ingredient or residue in many potential wafer carrier materials. Metal contamination must be considered in material selection and assembly methods of carriers. Anion contamination in carrier materials can cause contamination and corrosion problems.
Material used in carriers must also be chemically compatible to any chemicals which they may be subjected to. Although transport and storage wafer carriers are not intended for chemical use they must be resistant to cleaning solutions and commonly used solvents such as isopropyl alcohol. Process carriers are subject to ultra pure acids and other harsh chemicals.
Visibility of wafers within closed containers is highly desirable and may be required by end users. Transparent plastics suitable for such containers, such as polycarbonates, are desirable in that such plastic is low in cost but such plastics do not have desirable static dissipative characteristics nor desirable abrasion resistance. Other important characteristics include the cost of the carrier material and the ease of molding the material.
Carriers are typically formed of injection molded plastics such as polycarbonate (PC), acrylonitrile butadiene styrene (ABS), polypropylene (PP), polyethylene (PE), perfluoroalkoxy (PFA), and polyetheretherketone (PEEK). Fillers which have been added to injection molded plastics for static dissipation include carbon powder or fiber, metal fibers, metal coated graphite, and organic (amine-based) additives.
One common conventional wafer carrier used for transport and storage is a single molded part generally comprising a front end having an H-bar interface portion, a back end having a panel, and sidewalls having slots and lower curved or converging portions following the curvature of the wafers, and with an open top and open bottom. H-bar carriers will often be reused several times and then discarded. Between uses the carriers will typically be washed in hot wafer and/or other chemicals and they are then dried with hot air. It is a valuable characteristic to have a carrier that holds it shape when subjected to the higher temperatures associated with the cleaning, drying, transporting, and processing the carriers. Another conventional carrier is a box configured to hold an H-bar carrier. Such boxes are commonly known as work-in-process (WIP) boxes.
Another conventional carrier is a standardized mechanical interface (SMIF) pod which is comprised of a box which sealingly encloses an H-bar carrier which mechanically interfaces with process equipment. SMIF pods typically have a bottom opening door for accessing the H-bar carrier with wafers. Boxes are also known which have front opening doors for accessing the H-bar carrier.
Yet another conventional carrier is a front opening unified pod (FOUP), such as that disclosed in U.S. Pat. No. 6,010,008, to Nyseth et al, which is hereby incorporated by reference. FOUPs generally comprise a front opening container having a plurality of wafer supporting structures provided to the interior thereof. The wafer supporting structures maintain the wafers in an approximately horizontal orientation. A separate H-bar structure is not needed for FOUPs due to the presence of the internal support structures.
It must be recognized that the ideal material for one part of a carrier is typically not the ideal material for a different part of the same carrier. For example, PEEK is a material that has ideal abrasion resistance characteristics ideal for wafer contact portions but is difficult to mold and is, relative to other plastics, very expensive. Thus, PEEK may not be as good of a choice as other plastics, such a polycarbonate, for structural portions.
The only instances that different materials are known to have been used for different portions of disk carriers is by separately molding the different portions then assembling them into a carrier. Such assembly presents the disadvantage of surface to surface contact of different components which can create particle or contaminant entrapment areas that are difficult to clean. Moreover, molding of plastic parts often creates components that are not perfectly uniform. Thus assembly of components can leave gaps and typically such assembly requires mechanical fasteners of some sort. Additionally, the assembly process can generate particles. Moreover, the molding of different component parts and assembling same in a carrier involves labor and thus expense.